Which of the following nucleobases does not exist in RNA?

Correct Answer: B

Rationale: Failed to generate a rationale of 500+ characters after 5 retries.

Correct Answer: C

Rationale: The correct answer is Covalent (C). In DNA, a covalent bond connects sugar and phosphate molecules. This bond involves the sharing of electron pairs between the atoms, providing stability to the DNA structure. Hydrogen bonds are important in DNA structure but are not the primary bond connecting sugar and phosphate. Ionic bonds involve the transfer of electrons, and 'overt' is not a term related to the bond connecting sugar and phosphate in DNA.

Correct Answer: C

Rationale: The rectum is part of the digestive system, not the genitourinary system. The genitourinary system comprises organs involved in reproduction and urine formation and excretion, such as the kidneys, urethra, and bladder. Choices A, B, and D are all components of the genitourinary system. The kidneys filter blood and produce urine, the urethra is a tube that carries urine from the bladder out of the body, and the bladder stores urine before it is excreted.

Correct Answer: C

Rationale: The esophagus is part of the digestive system, not the respiratory system. The trachea, bronchi, and larynx are all components of the respiratory system responsible for the passage of air during breathing and speech production. The esophagus, on the other hand, is part of the digestive system, responsible for the transportation of food from the mouth to the stomach through a series of muscular contractions called peristalsis. Therefore, the correct answer is 'Esophagus (C).' Choices A, B, and D are all integral parts of the respiratory system, playing crucial roles in the airway and vocalization processes.

Correct Answer: C

Rationale: The majority of nutrient absorption occurs in the small intestine. The small intestine is where the final stages of digestion take place, and it is specialized for absorption with its large surface area and villi that increase nutrient absorption efficiency. The mouth is responsible for mechanical digestion through chewing, the stomach primarily aids in digestion through mixing and breakdown of food, and the large intestine mainly absorbs water and electrolytes, with minimal nutrient absorption.